Human joints can become damaged as a result of accident or illness. Such damage can be, for example, to the articular cartilage covering the ends of the bones at the joint as well as the intra-articular cartilage between the ends of the adjacent bones of the joint. When the damage to the joint is severe, a prosthetic joint can be implanted to improve the comfort and mobility of the patient.
Prosthetic joints have been developed to replace native tissue of several human joints. There are a variety of knee prostheses, hip prostheses, shoulder prostheses, ankle prostheses, elbow prostheses and wrist prostheses available to relieve patient suffering. Such devices are available, for example, from the assignee of the present invention, DePuy Orthopaedics, Inc. of Warsaw, Ind.
Standard prosthetic joints include metal components that are affixed to the articulating ends of the bones of the joint and commonly include a bearing component positioned between the metal components. Standard bearing components of prosthetic joints have a surface against which one of the metal components articulates. For example, hip endoprostheses include a metal femoral component to be affixed to the proximal femur and a metal cup to be affixed to the acetabulum. Many of these standard hip endoprostheses include a liner in the acetabular cup against which the femoral component articulates. Knee prostheses commonly include a femoral component to be affixed to the distal femur and a tibial component to be affixed to the proximal tibia. Bearings are typically between the femoral and tibial components.
An important consideration in the design, manufacture and implantation of any of these joint prostheses is adequate fixation of the bone-contacting prosthetic components to the native bone. Some designs of joint prostheses call for cemented fixation of prosthetic components to the native bone, using bone cement such as polymethylmethacrylate. Some designs encourage the ingrowth of hard tissue (that is, bone) around and onto the prosthetic component through the provision of porous outer surfaces on parts of the prosthetic component.
Even with the use of bone cement and porous surfaces, it is possible for a prosthetic joint implant to become loose over time. Loose bone-contacting components of a joint prosthesis (for example, stems received in the intramedullary canal) can become painful for the patient, and such loosening can eventually require revision surgery.
Movement of the bone-contacting components is a prognostic indicator of potential fixation failure through loosening (either implant to bone fixation, implant to cement fixation, or cement to bone fixation). However, accurate measurement of movement of bone-contacting prosthetic joint components can be problematic. Standard radiographs taken at different times (for example, immediately post-surgery and several months post-surgery) can be compared, but it would be difficult to detect small movements of the joint components accurately. More sophisticated techniques can also be used, but can be costly and can require special equipment. For example, radiostereometry analysis could be used, but this method requires that tantalum beads be implanted and requires software and expertise to obtain an accurate measurement of implant movement. It is believed that computer software is also being developed to measure implant migration through CT scans, but this method will also require additional equipment and expertise. In addition, the more sophisticated measurement techniques may expose the patient to additional radiation. Methods of evaluating implant loosening in the context of prosthetic hip joints are described in JOINT REPLACEMENT ARTHROPLASTY, 3rd ed. 2003, in Chapter 61, pp. 811-823, edited by Bernard F. Morrey, M.D., incorporated by reference herein.